In the past photographic determination of optical defects has been utilised,
1. a light source mounted alongside a camera objective; and PA0 2. a light source together with a fibreoptic light guide mounted in the camera objective.
In the first technique (Kaakinin's) the sensitivity is limited by the distance between light source and the optical edge of the camera lens used.
In the second technique (Braddick's) the sensitivity is limited by the thickness of the cladding which must surround the fibreoptic and also by the fibreoptic blocking off a portion of the lens.
In both Kaakinin's and Braddick's testing techniques an infant's attention is attracted to a flashing fixation light which is placed on or next to the camera objective and in some of the later experiments on the optic axis. Once the infant's attention is attracted an electronic flash is emitted from, one or two small flash tubes at right angles in Kaakinin's technique, or a single fibreoptic source mounted on the optic axis of the camera and in front of the objective in Braddick's technique.
In either case light travels from the source and is focused on the retina of the subject's eyes. If the eyes are optically good and are focused on or very close to the source then the retina image of the source is sharply defined. Light is reflected from two layers of the retina; the vitreo retinal interface and after passing through the retina a layer of brown pigment at Bruch's membrane. There is relatively little lateral diffusion of light in passing through these layers and so light reflected from both layers is passed back along the pathways by which it came to form a sharply defined image of the source exactly superimposed on the flash tube or fibreoptic from which it originated. In either case light from these layers is prevented from passing into the camera lens. In such a case a colour photograph taken of the subject's face will either show dark pupils or a dark red illumination of the pupillary aperture. This is due to the light passing through the retina and pigment cell layer and being widely scattered by the blood in the choroid before being reflected back towards the pupil from the inner layers of the sclera. This red reflex is very dim or absent in heavily pigmented eyes and relatively bright in blonde subjects but in all cases coloured a deep red by the choroidal blood and quite easily to distinguish from the lighter coloured reflexes from more superficial layers.
In the case of an eye which has a refractive error or which is not focused on the source of the light the retinal image is enlarged by being defocused so that the light reflected from the vitreo retina interface and Bruch's membrane forms an enlarged defocused image and is reflected back towards the source.
If this defocusing is sufficient for light to enter the edge of the camera lens (Kaakinin) or bypass the centre fibreoptic source (Braddick) then light from these superficial retinal structures enters the camera lens and produces a bright yellow or white appearance of the pupillary aperture in a colour photograph of the subject's face.
The sensitivity of both of these methods depends on the diameter of the subject's pupil and the angular distance between the light source and the clear edge (external Kaakinin or internal Braddick) of the camera objective. The sensitivity of the Kaakinin's technique is limited by the distance between the optic centre of the flash units and the optical edge of the camera lens employed. The sensitivity of the Braddick technique is limited by the thickness of the low refractive index cladding surrounding the fibreoptic source employed.
Both Kaakinin and Braddick's technique can be considered as crude forms of null test. Null tests are, in very broad terms, tests which in good optical systems when tested, allows no light to pass or else passes light evenly through all areas of its aperture.